There's iTunes the application, then there's your library. They are not interwoven. The application itself isn't large and is probably best left on the startup drive. You have various options when it comes to the library contents. It is not easy to split media storage by kind but if you are an orderly person it can be done. The easiest way is to delete the item from your library but do not allow iTunes to delete the file (your library just tells iTunes which files on your computer to play when you click a track). Now go to the media folder in your iTunes folder and copy the movie folder there to your external drive. Once it has put the files there, hold down the option key and drag the files to an iTunes window. This adds them back to the library but does not copy them to the internal hard drive. If all looks okay you can eventually delete the movie files on the internal.
The main disadvantage of doing it by the above method is you may lose things such as ratings, but if that isn't critical then it is the easiest method. Another disadvantage is if you do not have the external on when starting iTunes you will see a lot of ! indicating broken links, though this isn't permanent.
Another option is to move all of the iTunes library to an external drive. The main disadvantage of this is if you listen to music a lot you will have to have the external on and attached in order to do anything with iTunes. The solution above just moves movies and if you don't need access to them 100% of the time and can tolerate seeing a few exclamation marks then you only need attach the external when you want to watch a movie. Moving the whole library is what Niel is instructing.
iTunes: How to move [or copy] your music to a new computer [or another drive] - http://support.apple.com/kb/HT4527
Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the Option (alt) key (shift on Windows), then guide it to the new location of the library.
Message was edited by: Limnos